I had a chance the last couple of weekends to fish some during some evening hatches that lasted past dark. The problem is that after~8:45 -9:00 pm I could no longer see my fly (#18-20) elk hair caddis. I still got a few, but they were just lucky based on when I heard a splash.

To tell you the truth, it was rather difficult to see starting at ~8:15 w/ the sun at a steep angle. Any techniques that you employ during these times? Should I be switching from amber lenses to rose or yellow?

I used to fish the big flat above the Dam at Milesburg on Spring Creek PA. back before they took out the dam. After dark you have to position yourself so that the twilight in the sky is reflected on the water. This helps you to see the surface of the water if your spot is riffle water this may not work or if it is enshrouded by trees it won't work.

I'm not a good source to offer advice, but I can tell you what the guys at my FF club had me do. I told them how much trouble that I was having early in the morning and at dusk (more tying on flies; tying knots; managing line). They had me buy a light that clips on my hat. I haven't had to use it on the water yet.

pl4a,
I do a good deal of my trout fishing between dusk and dawn. I do not have a solution that will allow you to see your fly. I am 100% certain that you really do not want to be shining any light source over trout after the sun goes down.What you are doing now, using your hearing and instinct is the only answer I have. A thing to remember as well is that you can generally fish much closer to feeding trout in darkness.
I fish the Hex hatch here in Wisconson to the extent that I seldom am home for the two or so weeks that it is at its best. Many of the fish I catch take the fly at 15 feet or sometimes much less. I start casting an area before I lose light and try to guage the flys position as it drifts, commiting its position as well as I can to memory. If I suspect a take I simply lift the rod. If there is no fish struggling at the end of my tippet my lift sets up a backcast and I present the fly again.
Hex dries are pretty huge and though I often don't see the fly the takes are definitely audible. Caddis hatches seldom produce the noice level and consequently are inherently much more dificult to fish in the dark. I've tried tying flies with "glo in the dark" materials but gave that up because charging the phosphorescent materials with a bright head lamp just made conditions that much worse and one must be careful with bright lights or risk putting the fish down... My answer is to (this takes a bit of fortitude) take the caddis fly off, loop on some 1 or 2X tippet and change flys to a big streamer, frog or mouse. Big browns who spent the entire day securely tucked up under a log or undercut tend to make an appearance after the sun goes down and cruize about looking for larger meals. Sometimes the same trout who are busy dining on caddis as they let their guard down to feed. This mousing has become my favorite way to fish. I may spend several nights fishing for only a handful of takes but catch a fish that I never suspected was in residence on water I thought I knew well.
You should definitely know the water you are wading in darkness as a mis-step could be catostaphic at worst, cold and wet at best. Carry at least one extra light source to tie on flies, find your way out or in my case replace tippet to remove wind knots (smiles). But always use a light while facing away from the river if you are fishing a particular area.

For one thing, when it's dark, you can 'up' the size of your fly
It's not as important to match a size 18 bug with a size 18 fly, a 16 or even a 14 will do.
Also, use a fly tied with a wing that's easier to see. I use white antron myself, but a wing of pink or orange will practically light up like neon in low light.
Gary LaFontante's book Dry Flies has a chapter called Theory of Light (or something like that) that covers this well

Before it gets totally dark, like the 8:15-9:00 slot, I use yellow lenses. I bought them specifically for this purpose and I love them. The ones I got from Smith are the guides choice with the yellow lens. They are also my go-to glasses for stormy/cloudy/overcasts days. They really bring out the light.

when the sun goes past the horizon, i pretty much stop fly fishing. i find it too dangerous to fish because i take off my glasses(i always forget to pack a clear pair) and takedown is a bit of a hassle without a lamp.

go party, get drunk and pass out!!! hahahaha jk. well jk a lil... I have fly fished in the night I just use those yellow tinted hunting glasses to protect my eyes. I never caught nothing tho. better off getting a six pack and calling it a night! lol

Generally for me the sunglasses come off once the sun drops below the tree line. For some of the smaller flies try tying them with a black wing, it's surprising how well you can see it. After dark, as mentioned you can go up on size without too much problem; or you can toss big stuff.

I also fish the Hex hatch when I can and most of my flies are tyed with white deer belly hair for the wings, whether it is a paradrake pattern or a conventional dun pattern. I use the paradrake for duns and also for spinner falls.

If there are no Hex on the water, the mouse comes out...

Lights were mentioned, try not to use one; you lose your night vision for a short while after you use it. A friend has been using a small light with a red lens on it, he says he can see to tye on a fly and it doesn't affect his night vision.

As was mentioned, know the area you are fishing; you don't want to be wading an unfamiliar area at night.

Paul
Mice are a good Idea, I caught my biggest on a mouse, and streamers also produce well most will be black or lumo, but I have found even white works well, you shouldn't small tippet the fish are bigger at night

It is important too know the streach you are fishing, I use a small LED head lamp for my head it will go for 20 hrs and still give good light for tying the flies on, and I carry a spot light some times you can use it too spot fish in a hole and mark it for next time. feel and hearing is all you get you'll have trouble geting anything with your light on.

Ive tryed caddis after dark and you are guessing at best and will scare more than you catch